The correct answer is the expansion of slavery.
When the United States expanded westward through the era of Manifest Destiny, the question remained as to whether these new states would be free or slave states. The US Congress tried several different solutions.
The first one was the Missouri Compromise, which allowed Missouri to enter as a slave state and Maine to enter as a free state. Along with this, the 36'30 line established that any new state north of this line would be a free state. Any new state south of this line would be a slave state.
This held up for about 30 years, until the issue of adding California arised after the Mexican-American War. The Compromise of 1850 solves this problem temporarily. However, the fight over slavery expanding would continue in places like Kansas and Nebraska, ultimately leading to the Civil War.
